It has been necessary to employ exhaust gas flow-through catalyzed reactors for the conversion of carbon monoxide (CO), unburned hydrocarbons (e.g., propylene), and nitrogen oxides (NOx) in the exhaust gas stream flowing from, for example, gasoline-fueled, spark-ignited engines and diesel-fueled, compression-ignited engines that power automotive vehicles.
In the case of gasoline-fueled engines that are operated at close to the stoichiometric air-to-fuel mass ratio (e.g., about 14.5/1), a three-way catalyst, typically comprising platinum and/or palladium particles, carried and supported on alumina particles, has been effective in simultaneously oxidizing carbon monoxide to carbon dioxide, oxidizing residual hydrocarbons (HC) to carbon dioxide and water, and reducing NOx constituents (NO and NO2) to nitrogen and water. The exhaust gas, driven by the piston action of the reciprocating engine, flows through an extruded, monolithic, catalyst support body formed with many small, parallel, flow-through channels (e.g., 400 per square inch of inlet area) extending from an inlet face to an outlet face of the catalyst-support body. The walls of the many channels are suitably coated with a thin washcoat layer of the alumina-supported platinum-group metal catalyst particles. The relatively low oxygen content of the exhaust from the gasoline engine, operated in a stoichiometric air/fuel ratio combustion mode, typically permits the three-way conversion of CO, HC, and NOx during passage of the exhaust through a single catalyst body.
The exhaust stream flowing from a diesel-fueled engine or a gasoline engine operated in a lean-burn mode (A/F=14.6/1 or higher) typically contains about eight to ten percent by volume of oxygen and a like proportion of water. The temperature of the exhaust may vary from about 150° C. to about 500° C. depending on time of engine operation, current engine loading, and other variable engine operating conditions.
Compared with engines operated in a stoichiometric A/F mode, the lean-burn exhaust stream still contains small amounts of CO and HC, and it contains larger amounts of the NOx constituents. Each of these constituents must be converted in the exhaust to carbon dioxide, nitrogen, and water. And no single catalyst body has been devised to accomplish such conversions.
Different combinations of catalyst treatments have been proposed for the treatment of diesel exhausts. While a diesel engine is generally operated at a high A/F ratio (greater than 14.6/1) there are periods during cold start, heavy loading, or other driving situations in which the engine is briefly operated at close to stoichiometric A/F ratio, or even slightly fuel-rich (A/F about 14/1). The exhaust system must be devised to accommodate such engine operating modes. One system uses three or more catalytic reactors located in a progressive flow arrangement in the exhaust system, extending from the exhaust manifold of the diesel engine under the truck cab or passenger vehicle body, and extending to an outlet at which the treated exhaust is discharged to the ambient atmosphere. In this exemplary system, the exhaust gas flows sequentially through (i) a platinum and palladium-containing oxidation catalyst (DOC) for oxidation of CO and some HC, (ii) a lean NOx trap for storage of NOx during fuel-lean operation, and (iii) a selective catalytic reduction reactor (SCR) using ammonia as the reductant for NOx. An aqueous solution of urea is injected into the hot exhaust stream just upstream of the SCR reactor to provide ammonia for the catalyzed reduction reaction.
There is a need to reduce the costs of this complex exhaust treatment system for fuel-lean NOx containing exhausts. And one critical cost has been the use of platinum in the DOC. An alumina supported, platinum particle-containing catalyst has proven effective in the oxidation of CO and HC in diesel exhaust containing eight volume percent oxygen despite the wide temperature range (150° C. and higher) of exhaust temperatures produced during diesel engine operation. Palladium is much less expensive than platinum and is as active as platinum for CO and HC oxidation. But palladium tends to lose its activity to oxidize the CO and HC at lower exhaust temperatures (lower than 250° C.) after prolonged exposure to oxygen-containing exhaust at lean exhaust temperatures above about 300° C. It would be useful, and less expensive, to have a way to maintain the oxidation efficiency of a palladium-only DOC catalyst.